Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298 < T < 100 K).
Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298 < T < 100 K).
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低温（298 < T < 100 K）下 NaTiSi2O6 单斜辉石的单晶结构细化。
DOI：
10.1107/s0108768103022018
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
G. Roth
中科院分区：
文献类型：
--
作者：
G. Redhammer;H. Ohashi;G. Roth
在高温高压条件下合成了碱金属单斜辉石NaTi（3+）Si 2 O 6，并在298 ~ 100 K温度范围内对其进行了单晶X射线衍射研究。高温和低温形式之间的一个主要区别是在197 K以下，TiO 6八面体的无限链内突然出现两个不同的Ti（3+）-Ti 3+原子间距离。这种变化可以看作是在低温阶段形成Ti-Ti单线对的直接证据。平均Ti-O键长随着温度的降低而平滑地减小，相变与Ti-O键长的轻微跳跃有关。然而，对称性的破坏，导致不同的变化，特别是相对于两个Ti-O（顶点）键长，但也相对于四个Ti-O键在赤道面的八面体。的TiO 6八面体似乎是在链的方向上拉伸与稍大的伸长率在P1;低温相相比，C2/C高温相。多面体畸变参数，如键长畸变和八面体角方差表明，P1中的TiO 6八面体比C2/c更接近理想八面体的几何形状。平均Na-O键长随着温度的降低而降低，单个Na-O键长的变化是八面体位置几何形状变化的结果。四面体位点充当刚性单元，其在冷却和通过相变时不显示显著变化。对于四面体位置，当它们被绘制时，键长和键角以及多面体畸变参数都没有大的变化。与特别是在LiMSi_2O_6单斜辉石中发现的C_2/c → P_2（1）/c相变相反，没有发现四面体桥角的很大变化。因此，M1八面体位置是引起相变和控制结构变化的主要因素。
The alkali-metal clinopyroxene NaTi(3+)Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100 K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti(3+)-Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti-Ti singlet pairs in the low-temperature phase. Mean Ti-O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti-O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti-O(apex) bond lengths, but also with respect to the four Ti-O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P1; low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P1; to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na-O bond lengths decrease with decreasing temperature and the variations in individual Na-O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c-->P2(1)/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site.